import arcgisscripting, string, os, sys
gp = arcgisscripting.create(9.3)
gp.Workspace = r"U:\GIS\Temp"
ascList = gp.ListRasters("*.asc", "*")
for ascFile in ascList:
dsc = gp.Describe(ascFile) # Gets the Description about the ascii file
ascIn = dsc.CatalogPath # Gets the Full Catalog Path of the ascii file
ascOut = ascIn[:-4] + ".img" # Removes the .asc extension and adds .img
gp.ASCIIToRaster_conversion(ascIn, ascOut, "FLOAT")
ArcGIS doesn't see .asc files as files, they are recognized as raster datasets. arcpy.ListRasters() would have worked in this case.